1. Field of the Invention
The present invention generally relates to sample processing in a microwave oven and, more particularly, is concerned with an apparatus for dampening standing wave pattern generation in a closed cavity of a microwave oven.
2. Description of the Prior Art
Automated mechanical and bench (hands-on) methods of sample processing are the standard practice today in research and clinical laboratories. Though relatively time-consuming, these methods are reliable in the sense that they provide reproducible results.
Over the last two decades there has been a growing interest in accelerating the steps required in processing of samples, such as tissue and other specimens, for research and clinical applications. Interest has focused on the use of microwave ovens which emit radiation at 2.45 GHz +/-50 MHz. The presence of microwave energy and its heating effects, produced via ionic conduction and dipole rotation, provide increased rates of heat diffusion which reduce overall processing times compared to mechanical or bench methods.
Uneven microwave induced specimen heating, however, is a problem. The closed cavity design of microwave ovens produces standing wave patterns which are characterized by regions of high to low electric field density. These regions cause uneven heating of samples during microwave exposure and are referred to as hot and cold spots. The non-uniform heating of samples placed at different locations in the microwave cavity affects run-to-run reproducibility.
To overcome problems with use of microwave ovens, users must be particularly mindful to keep processing conditions (e.g. sample container, fluid volume, container placement in microwave cavity, sample number, same microwave oven) the same for each run. Users typically take steps to identify hot and cold spots prior to actual sample processing by use of the conventional methods of recognition, such as thermographic paints, neon bulb arrays and liquid crystal sheets, in order to be able to select the correct container placement location. Also, users have recirculated a dielectric fluid, such as water, around a confined sample to help control rapid heating during microwave irradiation.
Dummy loads, usually a beaker filled with water primarily serving as a heat sink, are frequently used in the microwave cavity during sample processing. Also, it is known that the use of multiple water loads will produce relatively large areas of uniform energy for multiple sample processing. Kok et al. (Ref. 1) have disclosed that a flat layer of cold water in the microwave cavity is effective in reducing hot and cold spots. The dielectric properties of water, an absorptive material, are important when water is used as a static dummy load. It is known that as water heats its dissipation factor decreases and the penetration depth of microwave energy into the water increases. Giberson et al. (Refs. 2 & 3) have disclosed that the benefits of recirculation and cooling of water within the microwave cavity via the dummy or water load have been demonstrated during specimen processing. Also, Giberson et al. have disclosed that when the temperature of water can be held constant the microwave environment remains constant and relatively large areas of uniform heating can be created.
Although the potential benefits of water recirculation and cooling in microwave processing of samples are thus recognized and appreciated, there still exists an unfulfilled need for a device that will bring about the realization of these benefits in practice.